1. Field of the Invention
The present invention relates to a system and device for using at least one servo channel to provide timing recovery and timing information to data channels.
2. Description of the Related Art
Magnetic tape cartridges include magnetic tape to store data to be saved and read back at a subsequent time. A magnetic tape drive writes the data to magnetic tape, typically as a set of parallel tracks, and subsequently a magnetic tape drive reads back the data. To read back the data, a magnetic tape drive typically comprises parallel read heads to read each of the parallel tracks, a drive system for moving a magnetic tape with respect to the read heads such that the read heads may detect magnetic signals on the magnetic tape, and a read channel for digitally sampling magnetic signals sensed by the read heads and providing digital samples of the magnetic signals sensed by the read heads. The digital samples are then decoded into data bits, and the data bits from the parallel tracks are combined to reproduce the data originally written on the storage medium. The read channel typically requires, among other signal processing functions, an equalizer for each of the read heads to compensate for the change in the signal characteristics due to the magnetic recording properties of the write head, the magnetic tape, and the read head. Magnetic tape cartridges may be interchanged between tape drives, such that a magnetic tape written on one tape drive will be read by another tape drive.
In recent years, the capacity and performance of tape storage systems has increased considerably, and the potential for further growth appears to be substantial. In order to achieve higher cartridge capacities and improved performance, advances in several technical areas are necessary. A real density increase, i.e. increase in linear and/or track density is key to achieving higher storage capacities. Increases in linear density result in a decrease in the distance between adjacent bit cells, which leads to an increase in intersymbol-interference (ISI). Higher track density requiring narrower track width, narrower write/read heads and closer head spacing, leads to losses in signal-to-noise ratio (SNR). Also issues of intertrack-interference are of greater concern. With increasing areal densities, accurate timing recovery on all parallel data channels during tape operation is critical for achieving reliable data retrieval.
In current tape systems, two dedicated servo channels may be provided to derive longitudinal position (LPOS) information as well as a lateral position-error signal (PES). The timing-based track-following servo for linear tape systems has been adopted by the linear tape open (LTO) consortium as a standard for the so-called LTO tape drive systems.
In a read-channel architecture where the analog data channel signals are synchronously converted into the digital domain, an analog-to-digital converter (ADC) is driven by a variable frequency oscillator (VFO) that may be controlled by a digital timing-recovery unit such that the readback signal is sampled synchronously with respect to the boundaries of the write clock operating at the rate of 1/T, where T is the nominal interval between consecutive timing samples. Typically, the rate of the write clock is chosen such that a predetermined recording density is achieved. The synchronous signal samples are first equalized and then provided to the detection circuit. Timing information may be extracted from the equalized sample values and decisions provided by the detection circuit. This architecture in the context of tape systems comprising M parallel data tracks requires M analog VFOs and their associated feedback control loops.
In a read-channel architecture where the analog data channel signals are asynchronously converted into the digital domain, the ADC is driven by a fixed clock with rate 1/Ts and the sampling of the readback signal is done asynchronously with respect to the write clock boundaries. The synchronization of the signal samples is accomplished digitally using interpolative timing recovery (ITR). No analog feedback loops and associated VFOs are needed, making this approach attractive for multi-track tape systems.
In the latter architecture, the ITR function can take place after or before signal equalization, leading to asynchronous or synchronous equalization schemes, respectively. The asynchronous equalization scheme leads to a relatively short timing-loop delay since the equalizer is placed outside the timing loop. In a synchronous equalization scheme, the equalizer is within the timing loop and therefore introduces additional timing loop delay. However, because the equalizer operates, in this scheme, on signal samples for which synchronization has been accomplished, adaptive equalization may be easier to achieve than with asynchronous equalization. As an example of a synchronous equalization scheme, in optical storage systems two interpolators may generate two sequences of synchronous even-time and synchronous odd-time samples which are equalized by means of two 2T-spaced synchronous equalizers before sequence detection.
With the current systems, timing recovery is performed by timing-recovery loops within each data channel that employ the interpolator output signal to perform the timing recovery operation individually for each data channel. The timing-recovery algorithms typically use equalized signal samples to determine the time instants at which signal sampling must occur.